1. Field of the Invention
The present invention relates to a liquid crystal composition.
2. Description of the Prior Art
In conventional twisted nematic (TN) liquid crystal display devices used as display elements for electronic wristwatches or the like, liquid crystals including those having a high N-I (nematic-isotropic phase transition) point are mixed in nematic liquid crystals (Np liquid crystals) having a positive dielectric anisotropy (.DELTA..epsilon.) so as to increase the N-I point. Such small liquid crystal devices are required to operate at a low voltage so as to simplify voltage source circuits and prolong the life of a battery.
The voltage for driving the display device must be sufficiently higher than a threshold voltage Vth of the liquid crystal material used. The threshold voltage is defined as the voltage applied to the liquid crystal material to change the amount of light (transmitted from the liquid crystal material) to a predetermined extent (i.e., 50%), at which the change in the transmitted amount is distinguishable with respect to the amount of light transmitted from the liquid crystal material when no voltage is applied thereto. The value of the threshold voltage is inherent to the liquid crystal material used. In order to drive the display device at a low voltage, therefore, a liquid crystal composition having a low threshold voltage must be used.
Conventional liquid crystal compositions, however, have high threshold voltages. A conventional liquid crystal composition must be driven at a voltage of no less than about 1.5 V even when it is statically driven. When the liquid crystal composition is dynamically driven, a sufficiently higher voltage must be applied thereto. For example, when a dynamically driven liquid crystal display device is time-divisionally driven at a 1/2 duty, a common connection electrode is selected for every half of one-frame period and an effective voltage applied between the segment electrode and the common electrode is decreased. For this reason, in order to dynamically drive a liquid crystal display device which has a conventional liquid crystal composition, a high voltage of about 3 V must be applied in view of the decrease in the effective voltage value.
In short, a statically driven liquid crystal display device using the conventional liquid crystal composition is driven by a battery having an electromotive force of 1.5 V at least. However, a dynamically driven liquid crystal display device using the conventional composition must be driven by two batteries each having an electromotive force of 1.5 V or a large battery having an electromotive force of 3 V. As a result, the liquid crystal display device becomes large in size as a whole. The conventional dynamically driven liquid crystal display device can be driven by one battery having an electromotive force of 1.5 V when the battery voltage is boosted. For this purpose, however, a booster complicates the arrangement of the driving circuit of the liquid crystal display device.
Furthermore, a threshold voltage and a viscosity of a conventional liquid crystal composition vary greatly in accordance with a change in temperature. At low temperatures, the threshold voltage is high and the composition cannot than be sufficiently driven at such temperatures, thereby degrading display contrast. Under the same conditions, the viscosity of the composition is increased to prolong the response time. As a result, conventional liquid crystal compositions can be used only within a narrow operation temperature range.